New insights in the understanding of the behaviour and performances of bismuth molybdate catalysts in the oxygen-assisted dehydration of 2-butanol

Pure Bi2Mo3O12, Bi2Mo2O9. Bi2MoO6. MoO3 and alpha-Sb2O4 and their mechanica l mixtures were investigated in the oxygen-assisted dehydration of 2-butano l at atmospheric pressure and at low temperature (220 and 250 degrees C). A ll catalysts were characterized before and after the catalytic reaction by BET surface area measurement, Raman spectroscopy, XRD and XPS. A strong par allelism is confirmed with the results obtained in the selective oxidation of olefins. In the frame of the remote control concept, alpha, beta and gam ma-bismuth molybdates are able to play a dual role: donor of spillover oxyg en (Oso) with respect to MoO3, and acceptor of Oso with respect to alpha-Sb 2O4. On one hand, this duality leads to mutual increase of activity when th e bismuth molybdates are mixed together. In the presence of MoO3, the alpha phase seems to be a stronger Oso donor than beta and gamma, and beta has a donor strength between alpha and gamma. On the other hand, when the Si mol ybdates are reacted in the presence of a big quantity of spillover oxygen, like in a mixture with alpha-Sb2O4, they undergo a dramatic decrease of act ivity. The phenomenon originates from the full oxidation of the reduced Mo species to Mo6+ induced by Oso. In parallel with other reactions involving oxygen, this confirms that the real active and selective state of molybdenu m-containing oxides is that slightly reduced possessing Mo5+. (C) 2000 Else vier Science B.V. All rights reserved.